Abstract: Within the framework of the plastic bounding surface model,and based on the structural damage theory and the unsaturated soil mechanics,an elasto-plastic model for unsaturated soils under cyclic loading has been elaborated.Firstly,on the basis of the Barcelona basic model,the water retention curve is employed to couple the suction and mechanical stresses under constant water condition.Secondly,the size of yielding surface is assumed to depend on the soil structure damage,and a law of structure degradation is given,linking the structure damage with accumulated plastic strain increments.Besides,by incorporating the concept of mobile mapping origin,the bounding surface model is modified to unify the mapping rule used during both loading and unloading so as to simulate the phenomenon of hysteretic properties under cyclic loading.Comparisons between experimental results or published data and model simulations show that the elaborated model is capable of describing the unsaturated loess behaviors under cyclic loading.